At its inception radio telephony was designed, and used for, voice communications. As the consumer electronics industry continued to mature, and the capabilities of processors increased, more devices became available use that allowed the wireless transfer of data between devices and more applications became available that operated based on such transferred data. Of particular note are the Internet and local area networks (LANs). These two innovations allowed multiple users and multiple devices to communicate and exchange data between different devices and device types. With the advent of these devices and capabilities, users (both business and residential) found the need to transmit data, as well as voice, from mobile locations.
The infrastructure and networks which support this voice and data transfer have likewise evolved. Limited data applications, such as text messaging, were introduced into the so-called “2G” systems, such as the Global System for Mobile (GSM) communications. Packet data over radio communication systems became more usable in GSM with the addition of the General Packet Radio Services (GPRS). 3G systems and, then, even higher bandwidth radio communications introduced by Universal Terrestrial Radio Access (UTRA) standards made applications like surfing the web more easily accessible to millions of users (and with more tolerable delay).
Even as new network designs are rolled out by network manufacturers, future systems which provide greater data throughputs to end user devices are under discussion and development. For example, the so-called 3GPP Long Term Evolution (LTE) standardization project is intended to provide a technical basis for radiocommunications in the decades to come. Among other things of note with regard to LTE systems is that they will provide for downlink communications (i.e., the transmission direction from the network to the mobile terminal) using orthogonal frequency division multiplexing (OFDM) as a transmission format and will provide for uplink communications (i.e., the transmission direction from the mobile terminal to the network) using single carrier frequency division multiple access (FDMA).
Another interesting feature of LTE is its support for multiple antennas at both the transmit side and the receive side. This provides the opportunity to leverage several different techniques to improve the quality and/or data rate of received radio signals. Such techniques include, for example, diversity against fading (e.g., spatial diversity), shaping the overall antenna beam to maximize gain in the direction of the target (beamforming), and the generation of what can be seen as multiple, parallel “channels” to improve bandwidth utilization (spatial multiplexing or multi-input multi-output (MIMO).
Precoding is a popular technique used in conjunction with multi-antenna transmission. The basic principle involved in precoding is to mix and distribute the modulation symbols over the antennas while potentially also taking the current channel conditions into account. Precoding can be implemented by, for example, multiplying the information carrying symbol vector containing modulation symbols by a matrix which is selected to match the channel. Sequences of symbol vectors thus form a set of parallel symbol streams and each such symbol stream is referred to as a “layer”. Thus, depending on the choice of precoder in a particular implementation, a layer may directly correspond to a certain antenna or a layer may, via the precoder mapping, be distributed onto several antennas.
Cyclic delay diversity (CDD) is a form of open-loop precoding in which the precoding matrix is intentionally varied over the frequency within the transmission (or system) bandwidth. Typically, this is realized by introducing different cyclic time delay for the different antennas, or alternatively realized by varying the phase of the transmitted signals from the different antennas. This kind of phase shift means that the effective channel, comprising the true channel and the CDD precoding, varies faster over frequency than the original channel. By distributing the transmission over frequency, this kind of artificially induced frequency-selectivity is useful in achieving frequency diversity.
One of the more significant characteristics of the radio channel conditions to consider in the context of high rate, multi-antenna transmission is the so-called channel rank. Generally speaking, the channel rank can vary from one up to the minimum of number of transmit and receive antennas. For example, given a 4×2 system as an example, i.e., a system with four transmit antennas and two receive antennas, the maximum channel rank is two. The channel rank associated with a particular connection varies in time and frequency as the fast fading alters the channel coefficients. Moreover, the channel rank determines how many layers, also referred to as the transmission rank, can be successfully transmitted simultaneously. For example, if the channel rank is one at the instant of the transmission of two layers, there is a strong likelihood that the two signals corresponding to the two layers will interfere so much that both of the layers are erroneously detected at the receiver. In conjunction with precoding, adapting the transmission to the channel rank involves striving for using as many layers as the channel rank.
FIG. 1 illustrates a transmission structure 108 for combining CDD and, possibly channel dependent, precoding. Therein, each layer 110 created by the transmitter presents a stream of information carrying modulation symbols to the CDD based precoder 112 as a sequence of symbol vectors 114. The CDD precoder 112 applies the two matrices 116 and 118 illustrated therein to each incoming symbol vector to perform the precoding process. More specifically, the CDD precoder 112 first applies the matrix UNT×r 118 to the symbol vector 114, followed by the diagonal CDD matrix 116. UNT×r matrix 118 is a column subset of a (possibly scaled) unitary matrix, r denotes the transmission rank and NT is the number of transmit antennas in the transmitting device. The notation Ak×l means a matrix A having k rows and l columns. The diagonal CDD matrix 116 has non-zero values along the diagonal including an antenna phase shift value θ indexed by a parameter k which may be a function of frequency. If OFDM is used for the transmission, k may e.g. represent the subcarrier index or the closely related data resource element index (which excludes resource elements containing reference symbols). It should also be noted that k may be a more arbitrary function of the position of the resource elements on the resource grid in OFDM. The resulting, precoded modulation symbol vector is then output for, e.g., resource mapping and OFDM modulation 120, prior to being transmitted via antennas 122 (also referred to as antenna ports).
The transmission structure 108 illustrated in FIG. 1 can be utilized in several ways. For example, one option is to use a fixed, channel independent, unitary matrix UNT×r 118 with a certain number of columns r corresponding to the transmission rank. The unitary matrix 118 serves to distribute each symbol on all antennas 122, while the diagonal CDD matrix 116 varies (shifts) the phase of each antenna 122. This increases the frequency selectivity of the effective channel each layer 110 experiences which, as mentioned above, can be useful for achieving frequency diversity (as well as multi-user diversity when frequency domain scheduling is used).
There are, however, certain problems associated with using the transmission structure 108 illustrated in FIG. 1 to perform precoding. The spatial correlation properties vary as a function of k and these variations need to be fast in order to ensure sufficient frequency diversity over even rather narrowband transmissions. This makes it difficult for a receiver to estimate the properties of interference stemming from such kind of transmissions. The transmission structure 108 also does not provide sufficient freedom to design the precoding onto the antenna ports. Furthermore, considering for example a r=1 rank one transmission, the transmission structure 108 will inherently use one column of the UNT×r matrix 118 to apply to the incoming symbol vector 114. This column would for example (in a two transmit antenna scenario) be equal to [1, 1]. Thus, this column together with the diagonal CDD matrix 116, forms a frequency selective beamformer which may be varied in a periodic fashion over the scheduled bandwidth. The period will depend on the selected speed of the phase variations. However, such beamforming may be problematic because, if the MIMO channel is correlated at the transmit side, severe cancellation of signals may occur at some frequencies. If the coding rate is not low enough over the scheduled bandwidth, this will in turn result in communication errors. Similar cancellation can occur even for transmission ranks greater than one. So, generally, it will be difficult to use high coding rates in conjunction with the transmission structure 108 (and its technique for precoding) if the scheduled bandwidth is over a significant portion of the previously mentioned beamformer period. Such a scenario, however, typically occurs when large delay CDD is used, i.e., corresponding to fast phase shift variations in the frequency domain.
Accordingly, it would be desirable to provide precoding systems, methods, devices and software which avoid the afore-described problems and drawbacks.